1. Field of the Invention
The present disclosure relates to a method of stopping an operation of a fuel cell system.
2. Discussion of the Background
A fuel cell system acquires DC electric energy according to an electrochemical reaction of a fuel gas (gas essentially containing hydrogen, such as hydrogen gas) and an oxide gas (gas essentially containing oxygen, such as air) respectively supplied to an anode electrode and a cathode electrode. This system is of a stationary type, or is mounted in a fuel cell vehicle as an on-vehicle fuel cell system.
For example, a solid polymer fuel cell has an electrolyte membrane/electrode assembly (MEA) having an anode electrode and a cathode electrode provided on the respective side of an electrolyte membrane formed by a polymer ion-exchange film; the electrolyte membrane/electrode assembly is sandwiched by a pair of separators. A fuel gas passage for supplying a fuel gas to the anode electrode is formed between one of the separators and the electrolyte membrane/electrode assembly. An oxide gas passage for supplying an oxide gas to the cathode electrode is formed between the other separator and the electrolyte membrane/electrode assembly.
When the operation of the fuel cell is stopped, supply of the fuel gas and oxide gas is stopped. However, the fuel gas remains in the fuel gas passage, and the oxide gas remains in the oxide gas passage. When the operation-stop period of the fuel cell becomes long, therefore, the fuel gas and the oxide gas may pass through the electrolyte membrane, so that the fuel gas is mixed with the oxide gas to react therewith, thereby deteriorating the electrolyte membrane/electrode assembly.
To cope with the problem, a fuel cell system disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-22487 (FIG. 1 and paragraph [0029]) shuts off the supply of a reaction gas to the anode side, and shuts off the supply of the reaction gas to the cathode side when the operation of the fuel cell is stopped. Further, the exhaust gas on the anode side is circulated to the upstream side through an anode-side circulation line, and the exhaust gas on the cathode side is circulated to the upstream side through a cathode-side circulation line, so that an electrochemical reaction in the fuel cell is maintained to generate power, thereby charging the battery with the generated power. Hydrogen in the exhaust gas on the anode side is consumed and oxygen in the exhaust gas on the cathode side is consumed this way, and a nitrogen gas is stored in a tank. The gases in the anode and cathode of the fuel cell are replaced with the nitrogen gas stored in the tank.